2.626984127 m
You have to know the equation that relates wavelength, frequency, and velocity (it's like speed but a bit different).
v = f x λ
where:
v = velocity
f = frequency
λ = Wavelength
Rearrange to make λ subject:
λ = v / f
We've been given 331 as the speed, 126 as the frequency. Sub it into the equation:
331 / 126 = 2.626984127 m